Tuned radio frequency (TRF) receivers were developed during the second decade of the twentieth century and patented by Ernst Alexanderson in 1916. FIG. 1 is a block diagram of a typical TRF receiver having an antenna 101 which provides radio frequency signals to a first tank or frequency select circuit 102, which provides a selected RF signal to a first RF amplifier 103. The amplified output of the first RF amplifier 103 is received by a second tank or frequency select circuit 104, which provides the selected and amplified RF signal to a second RF amplifier 105. The further amplified RF signal is received by a third tank or frequency select circuit 106, which provides the selected and further amplified RF signal to a detector 107, which demodulates the further amplified RF signal to produce an audio signal, which is sent to an audio frequency amplifier 108, and then to a speaker 109. The basic principle of the TRF receiver is that all tank (i.e., frequency selection circuits) are simultaneously tuned to the received frequency before detection and subsequent amplification of the audio signal. The dashed line 110 represents the ganging of the tuning capacitors of the three frequency selection circuits 102, 104 and 106. There are two significant disadvantages to the TRF receiver design. The first disadvantage is that selectivity is inconsistent over its tuning range. The receiver's ability to reject unwanted signals received by the antenna is a function of the receiver's selectivity. The second disadvantage is that it is difficult to make the multiple ganged stages track so that all change frequency by the same amount simultaneously. Consequently, most TRF receivers were equipped with multiple tuning knobs so that the frequencies of the tank circuits could be precisely tuned after an initial rough tuning.
In 1919, Edwin Armstrong developed the superheterodyne (also called super-sonic heterodyne or, simply, superhet) receiver, which was not beset by the disadvantages of TRF receiver cited above. FIG. 2 is a block diagram of a typical superheterodyne receiver. Radio frequency signals are received from an antenna 201 by a radio frequency amplifier 202. A mixer 203 receives amplified RF signals from the radio frequency amplifier 202 and a local oscillator signal from a local oscillator 204. The essential difference between a TRF receiver and a superheterodyne receiver is that in the former, the RF amplifiers preceding the detector are tunable over a band of frequencies, whereas in the latter, the corresponding amplifiers are tuned to a single fixed frequency called the Intermediate Frequency (IF). The IF is produced by the mixer 203, which combines the incoming RF signals with a signal generated by the local oscillator (LO) 204 at a frequency that will produce the IF. Thus, a desired incoming RF of known frequency is selected by choosing an LO signal of appropriate frequency. In this process, known as heterodyning, the two signals combine to produce an output beat frequency that is equal to the sum or difference of the original frequencies. The following relationship always holds true: the Intermediate Frequency (IF) is equal to the absolute value of the desired radio frequency (RF) minus the local oscillator (LO) frequency. If LO is greater than RF, this is termed high-side injection. If LO is less than RF, this is termed low-side injection. Through heterodyne action, any desired frequency within the receiver range may be converted to the intermediate frequency. The resultant beat frequency, which is at the IF, is sent to an intermediate frequency amplifier and filtering stage 205. The amplified and filtered IF signal is then sent to a detector 206, which demodulates the signal to produce an audio signal, which is sent to an audio frequency amplifier 207. An amplified audio frequency signal is then sent to a speaker 208. Thus, an incoming signal is converted to the fixed intermediate frequency before detecting the audio signal component, and the IF amplifier and filtering stage 205 operates under uniformly optimum conditions throughout the receiver frequency range. In a superheterodyne receiver, the IF circuits may be made uniformly selective, high in voltage gain, and of satisfactory bandwidth to contain all of the desired sideband components associated with a modulated carrier. Since the IF stages operate at a single frequency, a superheterodyne receiver may be designed to have better selectivity and sensitivity across the entire broadcast band and better gain per stage than a TRF receiver. Although a superheterodyne receiver typically contains more tuned stages than a TRF receiver, the majority of the stages are tuned to a single, fixed, intermediate frequency. This reduces the tracking problem and makes alignment of the tuned stages much easier. Unfortunately, sideband ambiguity inherent in the mixing stage of the superheterodyne receiver causes two closely related problems: double spotting and the reception of image frequencies.
“Double spotting” is a term which means that a desired station may be tuned in at two different selected frequencies. An image is an undesired IF intermediate frequency containing spurious information from a transmission on an unselected frequency. Rodney Champness, an Australian amateur radio enthusiast, clearly explains double spotting and image generation in Issue 171, titled Intermediate Frequency amplifiers, of his Internet publication, Vintage Radio. 
Mr. Champness uses the example of a superheterodyne receiver having an IF amplifier that operates at a frequency of 30 kHz. The local oscillator frequency must therefore be offset from the frequency of a desired station by the IF. As high-side injection is typically standard practice, if the desired station is transmitting at 800 kHz, the local oscillator will be tuned to 800+30=830 kHz. However, because selectivity of the RF stage of the receiver is generally not absolute, a station at a frequency of 860 kHz will also provide a 30 kHz IF output when mixed with a local oscillator frequency of 830 kHz. In the first case, the desired IF is generated by high-side injection; in the latter case, an undesired IF is generated by low-side injection of another RF signal at a different frequency in the RF reception band. This other RF signal is called the “image” because it and the desired signal are mirrored about (i.e., on opposite sides of and equidistant from) the LO signal frequency. If the undesired IF signal has a magnitude approaching that of the desired IF signal, it may jam or interfere with the latter. The amplitude vs. frequency plot of FIG. 3 shows the LO frequency, the desired signal's spectrum (RF) immediately to the left of the LO frequency, the undesired signal's spectrum (I) to the right of the LO frequency, and the resultant downconverted IF spectra at the far left after the downconversion mixing process. The downconverted desired signal (RFD) is superimposed on the downconverted image signal (ID).
To continue Mr. Champness' example, if the receiver is now tuned to 740 kHz, the oscillator will be on 770 kHz. However, this will also give a 30 kHz IF output from the 800 kHz station. Once again, we are dealing with high-side and low-side injection phenomena. This means that the 800 kHz station may be heard at both the 800 kHz and 740 kHz postions on the dial.
Given the fact that there are two RF signals that are frequency translated, or downconverted, to the IF by the mixer, it is desirable to filter out the undesirable frequency before it gets to the mixer so that it will not be frequency translated, or downconverted, to the IF frequency where it can jam or interfere with the desired signal. This is typically accomplished using what is called an image reject, or preselection, filter. Though the image reject filter is generally unable to completely eliminate the image signal, a properly designed filter will attenuate the image signal to a level where it does not cause appreciable interference with the desired signal at the IF after downconversion has taken place. The amplitude vs. frequency plot of FIG. 4 shows the LO frequency, the desired signal's spectrum (RF) immediately to the left of the LO frequency, the undesired signal's spectrum (I) to the right of the LO frequency, a bandpass filter having a passband (PB) represented by the window which encompasses both the desired signal spectrum (RF) and the LO frequency, and the downconverted IF spectrum (RFD) after the mixing stage. It will be noted that no downconverted image spectrum is present.
There are two competing design goals which mandate a compromise in the design of conventional superheterodyne receivers: that of facilitation of image rejection and that of channel selectivity. FIGS. 5 and 6 demonstrate how each design goal interferes with the other. In order to facilitate image rejection by the image reject filter, it is desirable to have a large frequency separation between the image frequency and the desired signal frequency. Clearly, if the image frequency and the desired signal frequency are too close to one another, then suitable image reject filter requirements may be unachievable, as the Q of the bandpass filter would be unrealistically large. Q is the Quality Factor of a bandpass or notch filter, and it is defined as the center frequency of the filter divided by the bandwidth. The bandwidth is defined as the frequency of the upper 3 dB roll-off point minus the frequency of the lower 3 dB roll-off point. Although one would initially suspect that the Q of a bandpass filter could be increased indefinitely, this is simply not the case. Although an in-depth discussion of bandpass filters is outside the scope of this discussion, a brief discussion is in order. Although the bandpass of the image reject filters of FIGS. 4 through 14 is depicted as a doorway having vertical (“brickwall”) side limits, this is only a convenient graphic representation that does not accurately represent the bandpass characteristics of real-world bandpass filters. As a practical matter, a single stage bandpass or notch filter is not uniformly effective over the entire bandwidth. As one might deduce from the above definition of Q for a bandpass or notch filter, the filter passband is most accurately represented as a bell-shaped curve. Thus, signals having frequencies near the center of the curve are passed most effectively, while those beneath the tails of the curve are partially attenuated. In order to construct a bandpass or notch filter having a bandpass approaching a rectangular, or brickwall, shape, it would be necessary to serially couple multiple filter stages. Another factor that needs to be considered with respect to bandpass filter shape is that of asymmetry. That is to say that instead of being a perfectly symmetrical bell curve, the passband curve may be shifted to either the right or left of center.
A large separation between the image frequency and desired signal frequency implies that the IF is large. The amplitude vs. frequency plot of FIG. 5 shows the LO frequency, the desired signal's spectrum (RF) well to the left of the LO frequency, the undesired, or image, signal's spectrum (I) well to the right of the LO frequency, a bandpass filter having a passband (PB) represented by the window which encompasses only the desired signal spectrum (RF), and the downconverted IF spectrum (RFD) after the mixing stage, which is spaced well to the right of the origin. Once again, it will be noted that there is no downconverted image spectrum. However, the problem with this scenario is that adequate channel selection is difficult to achieve, as the IF stage filter would require an unrealistically high Q. In this case, image rejection is enhanced at the expense of channel selectivity.
The conventional approach to superheterodyne receiver design is to use either high-side injection (most common) or low-side injection of the LO signal exclusively. However, even when the bandwidth of the image reject filter is larger than the (RF-IM) frequency, it is not possible utilize the entire filter bandwidth for image rejection when using only low-side injection exclusively or only high-side injection exclusively.
FIG. 6 demonstrates what happens when the IF is set to a low value in order to facilitate the selection of a desired channel from adjacent channels by an IF filter in the IF stage of the receiver. Because a low IF value implies a narrow frequency separation between the image frequency and the desired signal frequency, in this case channel selectivity is enhanced at the expense of image rejection. The amplitude vs. frequency plot of FIG. 6 shows the LO frequency, the desired signal's spectrum (RF) barely to the left of the LO frequency, the undesired signal's spectrum (I) barely to the right of the LO frequency, a bandpass filter having a passband (PB) represented by the window which encompasses both the desired frequency spectrum (RF) and the image frequency spectrum (I). In this scenario, the mixing stage results in a downconverted desired frequency spectrum (RFD), as well as a downconverted image frequency spectrum (ID), leading to almost certain jamming and interference.
Consequently, many contemporary superheterodyne receiver designs use multiple downconversion stages, with each stage having an image reject filter, a mixer, a local oscillator and a stage-specific IF, or multiple image reject filters, each having a different passband that is much smaller than the (RF-IM) frequency, and switching between those multiple image reject filters as a function of the desired channel frequency.
In order to overcome problems associated with the two competing goals of enhancing channel selectivity and enhancing image rejection, manufacturers of contemporary superheterodyne receivers typically resort to the use of complex design architecture. One common approach is to use multiple (usually two) downconversion stages, with each stage having at least one image reject filter, a mixer, a local oscillator and a stage-specific IF. Another approach is to use a single downconversion stage with multiple independent image reject filters, each of which has a different passband that is much smaller than the (RF-IM) frequency. Complex switching circuitry is employed to select between the various image reject filters as a function of the desired channel frequency. The downside to increased product complexity is increased design, manufacturing and warranty costs.
What is needed is a simple and inexpensive superheterodyne receiver architecture having both channel selectivity and image rejection that is on par with that of more complex and costly superheterodyne receiver designs.